DESCRIPTION: Catechol O-methyltransferase (COMT) has a major role in the inactivation of the estrogen of the estrogen catechol metabolites in peripheral tissues including breast tissue. COMT is coded for by a single gene that is polymorphic in the human population with 25% of Caucasians being homozygous for a low activity form of the enzyme. Given the accumulating evidence that catechol estrogens contribute to breast carcinogenesis and that COMT may play a protective role, it is hypothesized that women homozygous for the low activity form of COMT would be at increased risk for breast cancer. Using a PCR-based RFLP assay developed in the applicant's lab, 116 cases plus 116 matched controls were screened from a prospective case-control study nested within a large (30,000) cohort study from Western Maryland. The results show that the low activity COMT genotype confers a significantly increased risk for breast cancer in postmenopausal women (odds ratio=2.3; p=0.05). This hypothesis-driven molecular epidemiology study is the first to suggest that a polymorphism in an enzyme involved in the inactivation of a reactive estrogen metabolite is associated with an increased risk for breast cancer. The goal of the project described in this application is to conduct a rigorous experimental investigation of the hypothesis that decreased COMT activity results in increased oxidative damage and this stress that with time contributes to increased cell transformation and breast cancer. The specific aims of this project are: 1) to determine the effects of COMT inhibition on oxidative damage and stress in selected human breast epithelial cell lines expressing the wild-type (MCF-10F) and variant (MCF-7) forms of COMT; 2) to determine the kinetics of the wild type and variant forms of COMT in cytosols from these cell lines for methylation of the 2-OH and 4-OH catechols of estradiol; and 3) to determine the effects of COMT inhibition on breast cancer in vivo using a COMT-ribozyme transgenic mouse.